1. The Field of the Invention
The present invention relates to the formation of a contact for an integrated circuit device on a semiconductor substrate, such as a silicon wafer. More particularly, the invention is directed to the formation of a self-aligned contact for a memory device in an integrated circuit device formed on a semiconductor material layer or substrate.
2. The Relevant Technology
As microchip technology continues to increase in complexity and decrease in component size, dimensions are shrinking to the quarter micron scale and smaller. With use of the current high-yield photolithographic techniques, the margin of error has become increasingly tighter such that a single misaligned fabrication step can cause an entire chip to be flawed and be discarded. As devices shrink further, overstepping each process step""s window of error increases the likelihood of fabrication failure. A production worthy device feature requires incidental skill of a process engineer and a fabrication operator to fabricate the feature.
One device that is subject to the ever-increasing pressure to miniaturize is the dynamic random access memory (DRAM). DRAMs comprise arrays of memory cells which contain two basic componentsxe2x80x94a field effect access transistor and a capacitor. Typically, one side of the transistor is connected to one side of the capacitor. The other side of the transistor and the transistor gate electrode are connected to external connection lines called a bit line and a word line, respectively. The other side of the capacitor is connected to a reference voltage. Therefore, the formation of the DRAM memory cell comprises the formation of a transistor, a capacitor and contacts to external circuits.
It is advantageous to form integrated circuits with smaller individual elements so that as many elements as possible may be formed in a single chip. In this way, electronic equipment becomes smaller, assembly and packaging costs are minimized, and integrated circuit performance is improved. The capacitor is usually the largest element of the integrated circuit chip. Consequently, the development of smaller DRAMs focuses to a large extent on the capacitor. Three basic types of capacitors are used in DRAMsxe2x80x94planar capacitors, trench capacitors, and stacked capacitors. Most large capacity DRAMs use stacked capacitors because of their greater capacitance, reliability, and ease of formation. For stacked capacitors, the side of the capacitor connected to the transistor is commonly referred to as the xe2x80x9cstorage nodexe2x80x9d, and the side of the capacitor connected to the reference voltage is called the cell plate. The cell plate is a layer that covers the entire top array of all the substrate-connected devices, while there is an individual storage node for each respective storage bit site.
The areas in a DRAM to which an electrical connection is made are the gate of a transistor of the DRAM, a contact plug to an active area, and the active area itself. Active areas, which serve as source and drain regions for transistors, are discrete specially doped regions in the surface of the silicon substrate. A bit line contact corridor (BLCC) is created in order to make electrical connection to an active area. The BLCC is an opening created through the insulating material separating the bit line and the active area. The BLCCs are filled with a conductive material, such as doped polysilicon, doped Al. AlSiCu, or Ti/TiN/W. Before filling the BLCC, however, a process engineer must design a process flow for fabricating the BLCC that assures that the BLCC is not misaligned, and therefore not prone to shorting out or subject to errant charge leaking due to an exposed cell plate in the BLCC.
Conventional methods of fabricating bit line contacts may tend to cause shorting of the bit line contact in the BLCC into the cell plate due to misalignment. For example, titanium is conventionally sputtered into a BLCC. Next, titanium nitride is deposited by CVD or PVD processing. A rapid thermal anneal step (RTA) then causes silicide formation. Tungsten is then deposited to fill the remaining opening in the BLCC. Depending upon the accuracy in the formation of the BLCC itself, it is possible of the BLCC to be shorted to other conducting layers. This is described below. In general, the BLCC can also be composed of tungsten, titanium/tungsten, aluminum, copper, a refractory metal silicide with aluminum, and a refractory metal silicide with copper.
As the size of the DRAM is reduced, the size of the active areas and the BLCCs available for contacts to reach the active areas are also reduced. Every process step has its own alignment limitations. While alignment is not exact between process steps, strict tolerances are required in order to accomplish a corridor that avoids a short between a contact that will be deposited in the BLCC and any other conductive materials (i.e. cell plate to active area). Hence, it is desirable to effectively isolate the contacts from the transistor and capacitor components while optimizing the space available to make the contacts.
The conventional methods of forming contacts between bit lines and an active areas experience alignment problems in avoiding a short circuit between the electrically conductive bit line contact and the cell plate or storage node of a capacitor.
A method and structure is disclosed that are advantageous for preventing shorting of a contact to an active area with a capacitor cell plate and a capacitor storage node. In accordance with one aspect of the invention, a method of fabricating a DRAM is disclosed that utilizes an insulated sleeve structure to self-align a bit line contact corridor (BLCC) to an active area of a DRAM transistor. In accordance with this aspect of the invention, capacitors are formed over a semiconductor substrate. In the context of this document, the term xe2x80x9csemiconductor substratexe2x80x9d is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term xe2x80x9csubstratexe2x80x9d refers to any supporting structure including but not limited to the semiconductor substrates described above.
In the inventive method, a lower bulk insulator layer is formed upon the semiconductor substrate, and a dielectric layer is formed upon the lower bulk insulator layer. Next, a conductor layer is formed upon the dielectric layer and an upper bulk insulator layer is formed upon the conductor layer. An etch is performed to selectively remove the conductor layer, the dielectric layer, and the lower bulk insulator layer so as to form an opening defined by the lower bulk insulator layer, the dielectric layer, and the conductor layer. The opening terminates at a bottom surface within the lower bulk insulator layer above the semiconductor substrate.
Next, a sleeve insulator layer is deposited upon the upper bulk insulator layer and within the opening so as to make contact with each of the lower bulk insulator layer, the dielectric layer, and the conductor layer. An etch process is then performed to substantially remove the sleeve insulator layer from the bottom surface within the lower bulk insulator layer above the semiconductor substrate, and from on top of the insulator layer, thus leaving the sleeve insulator layer in contact with each of the lower bulk insulator layer, the dielectric layer, and the conductor layer.
Another etch process then selectively removes the lower bulk insulator layer to create a contact hole defined by the sleeve insulator layer and the lower bulk insulator layer and to expose a contact on the semiconductor substrate. A conductive plug is then formed in the contact hole upon the contact on the semiconductor substrate such that the sleeve insulator layer electrically insulates the conductive plug from the conductor layer.
The sleeve insulator layer, which self aligns the BLCC, allows for improved alignment tolerances between the BLCC and other layers, thus preventing errant charge leakage and short circuits between the conductive plug formed within the BLCC and the other layers.
Conceptually, the etching of the BLCC progressively deeper into the lower bulk insulator layer can be carried out incrementally with a plurality of depositions of the material of the sleeve insulator layer, each said deposition being followed by an etch of the sleeve insulator layer to remove the same from the bottom of the BLCC within the lower bulk insulator layer.